A mask is a structure used in photolithography to place a desired image, typically an image of a layer of circuitry, onto a surface of a photoresist layer. Light is selectively passed through the mask and exposes the surface of the photoresist layer with the desired image such that the photoresist layer can be selectively removed, thereby enabling the processing of underlying semiconductor material to form an integrated circuit. Traditional masks include a transparent substrate, e.g., quartz, and light blocking and light passing features formed in an optically opaque material such as chrome. Phase shift masks extend the mask art by providing the ability to alter the phase of light passing through the mask thereby enabling various optical effects, such as constructive and destructive interference, to be used during the exposure of the photoresist. The exposure “light” or radiation may be in the visible regime, in the ultraviolet (UV), deep ultraviolet (DUV), or extreme ultraviolet (EUV) regimes and beyond.
Recently the mask art has been further extended by the introduction of electronically programmable masks. For example, electronically programmable masks are described in U.S. Pat. Nos. 5,998,069 and 6,084,656 which typify a type of photomask that employs a technology similar to that of a liquid crystal display (LCD). The mask is provided with an array of programmable pixels made of optical shutter devices. By a polarization process of liquid crystal material of the photo mask light can be selectively transmitted through portions of the mask. In addition, by using this approach different degrees of phase shifting of the transmitted light can be achieved.
More specifically, U.S. Pat. No. 6,084,656 describes several different approaches to making a programmable mask for optical exposure. A first approach uses a liquid crystal (LC) filter sandwiched between two optically transparent electrodes. Photons coming through a back electrode at selected LC pixels can be either blocked by or passed through the liquid crystal medium and exit to the front electrode, depending on whether the pixels are electrically biased or not. The voltage variable birefringence properties of liquid crystal material are well documented in the literature. A second approach uses an array of micro non-transparent bottom electrodes as optical shutters. These electrodes are electrically actuated. A third approach uses a micro-reflecting plate to deflect light at pixel-level resolutions. The micro-reflecting plate can be activated by an electrostatic or by a thermal expansion force.
U.S. Pat. No. 5,998,069 describes an electronically programmed photolithography mask based on a single array of optical filters, or on a stack of arrays of optical filters, having transmission characteristics whereby the opacity to and polarization of an incident light beam can be modulated electrically. Like U.S. Pat. No. 6,084,656, liquid crystal material is disclosed for use as the filter medium. An advantage of using a stack of liquid crystal filters is that it provides more discrete control of opacity and polarization at the pixel level in order to simulate various intensity and phase shift effects.
Another example of a programmable mask is described in U.S. Pat. No. 6,060,224, which employs an array of addressable and rotatable micro-mirrors. The light can be selectively reflected from certain surfaces of the micro-mirror array to produce useful patterns. As the micro-mirrors are addressed they rotate to reflect light from a remote source onto a portion of a photoresist coated wafer. Multiple layers of micro-mirror arrays can be interleaved to increase the spatial resolution of the resulting transmitted image. In the case of an EUV light source in the range of 4.5 nm to 15 nm wavelength, the micro-mirror elements may be composed of multilayer Bragg reflective coatings that reflect the EUV radiation.
There are at least two major disadvantages in the use of these conventional programmable masks. The first disadvantage is that they require an external light source. This implies that a projection facility and wafer exposure and transporting station are required, as in conventional full-sized conventional lithographic tools. This then further implies an increased cost and complexity, an inflexibility in the exposure scheme, and that the lithographic tool is bulky and difficult to maintain. The second disadvantage is that the heat that absorbed by the mask may damage the electronic devices built into the mask. The accumulated heat may also cause the quality of the mask to be degraded. In other words, the micro-mirrors, or the opaque portions of the liquid crystal mask, will absorb a substantial amount of heat during exposure to the light source. If the heat is not properly dissipated, it can cause the mask to warp, and/or it can induce a mechanical malfunction in the micro-mirror mask. The heat may also degrade the quality of the liquid crystal material.